Sound and science

Jean-Louis Migeot argues that acoustic simulation needs to be an early part of any product design and that software is now available to ensure it can be done easily.

Sound is often an indicator of function and quality. Sometimes it’s silence – like a dishwasher you can run at night without keeping the kids awake. Sometimes it’s the reassuring ‘thunk’ of a car door closing solidly. Whatever the product, making the expected sound is part of what helps it succeed or fail.

With the advent of new materials and design imperatives, companies can’t take their products’ sound profiles for granted. They need to know in advance whether the shutter click of the digital SLR camera sounds enough like a mechanical SLR to satisfy serious photographers. They need to know their hybrid vehicle is so quiet, when it’s running on battery power, that drivers can hear gasoline sloshing in the tank, for they need to mask that noise.

Creating the right sound profile for a product is increasingly complicated. The current emphasis on energy efficiency and smaller environmental footprints, driven by government mandates and consumer preferences, influences the design of vehicles, aircraft, personal electronics and household appliances. Integrating acoustic simulation software into their design processes gives them insight before their products hit the showroom floor – or in other words, when it’s too late.

But few companies have the tools and processes to incorporate new materials and designs into their products without altering their sound profiles for the worse.

Efficiency versus noise

The conflict between efficiency and noise is perhaps most stark in the aerospace industry. Aircraft manufacturers face strong pressure to make aircraft both quieter and more efficient, even though those two imperatives often conflict. Giving one a lower priority won’t work, because they both directly affect the aerospace industry’s economics.

Noise probably has a more profound influence on air travel than on any other industry. Airports around the world restrict take-off and landing hours to protect surrounding areas from excessive noise. At the same time, air travel’s popularity and the world economy’s reliance on overnight air freight are creating demand for more flights. Unless the aerospace industry responds to the noise challenge with quieter aircraft, local authorities will have no choice but to manage airport noise by restricting hours.

But fuel costs account for anywhere from 29 to 41 per cent of airline budgets, and the prices keep rising. So, while responding to noise concerns, aircraft manufacturers also have to keep fuel efficiency high on their priority list.

Aerospace engineers have developed methods for increasing fuel efficiency. Light new materials reduce aircraft weight and fuel consumption. Streamlined new fuselage shapes reduce air friction. Advanced engine designs, such as counter-rotating jet engines, consume less fuel than conventional engines.

Each of these fuel-saving measures has noise implications. For engineers to balance noise and fuel-efficiency, they must be able to predict how the aircraft will perform in the physical world, long before it becomes a physical object as a prototype. This is the big weak spot in most product design processes. Engineers tend not to be provided with acoustic simulation tools that will help them understand, early in the design process, whether a fuel-saving modification will make the aircraft noisier.

Early is critical. Waiting until later in the process limits engineers’ ability to make substantial changes and adds significant cost and time to the development process.

Integrated acoustic simulation

Airbus is the successor to the consortium that developed the noise-plagued Concorde supersonic jet, so it has a long history of managing jet noise.

About 15 years ago, Airbus and jet engine maker Rolls-Royce teamed up with Free Field Technologies to develop Actran – acoustic simulation software that can model entire systems, i.e. the combination of engine noise and airflow around the fuselage.

The earliest acoustic simulation software was too sophisticated for every engineer in the process to learn, but Airbus has worked steadily over the past 15 years to ‘democratise’ it. Today, acoustic simulation is fully integrated into Airbus’ design processes.

Any design engineer in Airbus can initiate an acoustic simulation calculation. They change values for parameters such as speed, temperature and altitude in a simulation model and submit it for calculation. Actran runs the simulation and reports the results. Airbus engineers can use the software at the beginning of the process to get a broad idea of which geometry is most promising.

As they get closer to a final design, they can adjust the parameters for optimal performance. This system eliminates guesswork and needless iteration. It avoids costly, late-stage errors, through constant simulation that reveals when an idea is going awry.

Whatever a company’s sound management challenges, Airbus shows that it is possible to infuse acoustic simulation into design processes from beginning to end.

Other companies might not have to simulate every phase of the process, because their products do not change significantly from one version to the next. They might need only occasional acoustic simulation to verify their sound profile. Or they might be able to create one acoustic model of their product, and then use it to create best practices for sound engineering. The bottom line is that companies don’t need to have the resources of a large aerospace company to embrace acoustic simulation, but they do need to embrace acoustic simulation if they are to deliver function and quality, as well as the sound profile that acts as an indicator of that function and quality.

Dr Jean-Louis Migeot is co-founder and CEO of Free Field Technologies, an MSC Software company.

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